2 Guidelines for Levelling DFDs External communication: external agents represent entities in the environment of our information system external agents are outside the scope of our information system we do NOT model interactions between external agents we do NOT allow external agents to interact directly with data stores Guidelines for Levelling DFDs numbering: when a process is decomposed, its diagram is given the same number as that process balancing of levelled DFDs: all data flows entering and leaving a process must appear on the corresponding diagram which decomposes that process external agents: are only included on the two diagrams which represent the entire system, i.e. the context and level zero diagrams Guidelines for Levelling DFDs the access to data stores across levels of diagrams must be consistent: the direction of accesses must match and all accesses on higher level diagrams must appear on corresponding lower level diagrams a data store is first shown on the highest level diagram where it is accessed by more than one process it can then appear on all lower level diagrams where it is accessed 2

3 Guidelines for Levelling DFDs the access to data stores across levels of diagrams must be consistent: Guidelines for Levelling DFDs How many levels should be in a set of DFDs? each diagram usually has between 3 and 7 processes level the diagrams until bottom level or primitive processes are reached: primitive processes have only 1 or 2 inputs and outputs, and cannot be further decomposed as a data flow diagram Guidelines for Levelling DFDs partition processes to minimise the data flows between them partition processes to form cohesive, related groups of activities not all parts of the system may need to be decomposed to the same level 3

6 Logical and physical DFDs Models may focus on either: the physical view of the real world how things are done OR the logical view of the real world what things are done Physical DFDs represent a particular way of implementing the processes and data in a system they are technology dependent they specify particular methods of doing tasks they show how the processing takes place and how the data is implemented Logical DFDs represent what a system must do regardless of how it is implemented they are technology independent they show what processing, data movements and data storage must occur in a system they show the essential aspects of a system 6

7 Using Logical and Physical DFDs Physical DFDs modelling current system: help systems analysts become familiar with how a business or system operates Physical DFDs modelling new systems: model the technical and human design decisions to be implemented users can relate to physical DFDs more readily because they contain implementation details: landmarks e.g. people or roles, actual locations Use of Logical and Physical DFDs Systems analysts often begin with physical DFDs of current systems: convert that physical DFD to a logical model in order to focus on essential elements use the logical model to model a new logical solution (DFD) Convert the logical solution into a physical DFD (implementation) model implementation details can be removed from physical DFDs Physical to Logical DFDs use names for data flows and data stores which indicate their content, not their physical form or location use names for processes that indicate what, not how 7

10 Function Decomposition: a Process Modelling Technique function decomposition is the decomposing of a system into its component functions and processes as a way of managing complexity function decomposition creates a top-down view: it shows a hierarchy of levels of increasingly detailed processes within a system Function Decomposition: a Process Modelling Technique a function decomposition model (or diagram) is used to represent the hierarchical decomposition and structure of the processes of a system the decomposition of functions corresponds to levelling of processes in DFDs Function Decomposition Diagrams a function is a high-level set of related activities that are ongoing: a function is a broad, generic activity a process is a lower level activity that is repeatedly carried out functions consist of groups of related processes the depth and scope of function decomposition diagrams depend on the size and complexity of the system represented 10

11 Function Decomposition Diagrams function decomposition diagrams showing only higher levels can be built early in systems analysis more detailed process decomposition can be carried out as the system is studied in more detail function decomposition diagrams show the structure of functions and processes within the system this structure may not correspond to the structure of the organisation shown in an organisation chart See Whitten et al (2001) p 5, p 338 for examples Function Decomposition Diagrams each function and process should have a unique name which indicates what it does ABC Sales System 1. Sell Products 2. Manage Inventory 3. Control Finance a top level function decomposition diagram Example Function Decomposition Diagram ABC Sales System 1. Sell Products 2. Manage Inventory 3. Control Finance 2.1 Deliver Product 2.2 Accept Delivery 2.3 Check Stock levels functions and processes are further decomposed 11

12 Function Decomposition the decomposition of functions and processes corresponds to the levelling of DFDs Function Decomposition function decomposition diagrams are an alternative representation of the hierarchy of functions and processes within a system they may be built using either a top down or a bottom up approach they provide a useful overview of the processing within a system Process modelling principal techniques function decomposition data flow diagrams associated techniques for modelling the details of low-level processes (next week) structured English decision tables and decision trees 12

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